Passive adsorption microvacuum pump and microsystem containing the same

ABSTRACT

A micro-absorption pump can be used to form microsystems with microdevices such as a mass spectrometer, micro chromatograph, micro crack sensing devices which can be used in microcrack detection of large aeroplane and other structures and chemical or gas sensing lab-on-chip devices, which need evacuation prior to their use. The micro-absorption pump has a chamber containing a cooled absorbent material which is capable of retaining gas molecules on a surface and reducing the pressure of the atmosphere containing the gas molecules. The absorbent is cooled by a liquid, such as liquid nitrogen, or a peltier device and the absorbed gas can be released by heating the absorbent. The micro-absorption pump can be connected to the microdevice to create a low-pressure environment therein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a passive micro-sorption pump which utilizes an adsorbent to effect a pressure reduction in a microsystem and to a microsystem made up of the micro-sorption pump and a microdevice.

2. Description of the Related Art

Sorption pumps which utilize an absorbent material to retain gas molecules on its surface are widely used in obtaining vacuums at a pressure of down to 10⁻² torr and have been used in series to produce a vacuum as low as 10⁻⁵ torr. The adsorbent materials typically are molecular sieves that have been processed so that they are porous and have pore sizes comparable to the size of the molecules to be absorbed. The adsorbent is typically positioned inside a cylindrical chamber that is connected to a vacuum system and can be immersed in liquid nitrogen for cooling to those temperatures, which aids the adsorption process. These types of pumps have been used mainly for roughing systems in which the sputter ion and titanium supplemation pumps serve to insure freedom from organic contamination.

An article by Richard L. Barber et al in J. Micromech. Microeng. discusses design rules for fabricating microdevices having an integrated micro-sorption pump for vacuum generation. This article discloses the use of a nanometer-sized thin film, particle free getter used in a micropump which operates by adsorbing the gases, and binding them to the surface of the getter and creating a reduced pressure in the packaged microdevice. This article additionally discusses general design considerations, which must be evaluated in the fabrication of a microdevice having an integrated micro-sorption pump but does not disclose the specific configuration or operation of such a micro-sorption pump and a microsystem containing the pump. To date, there has not been proposed a practical working model of a micro-sorption pump which can be utilized in a vacuum-integrated microdevice.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a micro-sorption pump which can be incorporated into a vacuum-integrated microdevice. The micro-absorption pump has a high-aspect-ratio chamber made of a thermally conductive material and an adsorbent material contained therein. The micro-sorption pump can have an outer chamber which can contain a suitable cooling material or can be integrated with a micropeltier device. The micro-sorption pump also can be integrated with a microdevice through the use of a mounting sleeve and communicates with a micro-channel provided in a microdevice to provide an integrated microsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a micro-sorption pump according to the present invention;

FIG. 2 is an exploded view of an integrated microsystem according to the present invention;

FIG. 3 is a sectional view along section lines 3-3 of FIG. 2; and

FIG. 4 shows a micro-sorption pump of the present invention integrated with a microdevice.

DETAILED DESCRIPTION OF THE INVENTION

As illustrated in FIG. 1, a first embodiment of a micro-sorption pump 1 of the present invention comprises a base wall 2, a first continuous vertically-extending wall 3 provided on the base wall 2 and enclosing a first space therein, a second continuous vertically-extending wall 6 provided on the base wall 2 inside the first continuous vertically-extending wall 3 and which encloses a second space 8 therein and cooling fins 10 which are provided on the second continuous vertically-extending wall 6 and extend into the second space 8. A chamber 7 is formed between the first continuous vertically-extending wall 3 and second continuous vertically-extending wall 6 and serves as a reservoir for containing a liquid coolant or a space which can be integrated with a micropeltier unit 11 for cooling the second continuous vertically-extending wall 6 and thereby cooling the fins 10 and an adsorbent contained in the second space 8.

The micro-sorption pump 1 is made of any metal, alloy or semiconductor with reasonable thermal conductivity, such as copper, aluminum, nickel, alloys thereof, nickel-iron alloys, and silicon, etc., are suitable as a material of construction for the pump. The metallic micropump structures (e.g. copper or nickel) can be fabricated on a silicon wafer by first making the structures using Lithographie, Galvanoformung and Abformung (LIGA) manufacturing technology. If silicon is chosen as the material for micropump structure, this can easily be fabricated using Deep Reactive Ion Etching (DRIE) or even more conventional wet etching technologies (KOH etching, TMAH etching or EDP etching) depending on the minimum feature dimensions considered in the design. LIGA and DRIE processes are well known in the art for fabricating microstructures. In the present invention, LIGA can be used to expose an x-ray sensitive thick polymer layer through a designed mask to make a mold for electroplating the high aspect ratio microvacuum pump of the present invention or the structure itself can be used as a pump. On the other hand, instead of using a LIGA fabricated mold, a DRIE etched silicon mold can be used for electroplating. Alternatively, a silicon mold structure can be prepared by using DRIE or wet anisotropic etching as mentioned above. All of these processes are well known to one of ordinary skill in the art.

The micro-sorption pump 1 is a monolithic structure and the base wall 2, first wall 3, second wall 6 and cooling fans 10 are all made of the same thermally conductive material. While the wall thicknesses, number of fins and shapes of fins have little effect on the thermal profile inside the second space 8, these parameters were found to have an effect on the structural integrity of the device as a whole. It was found that thicker wall chambers reduce the stress gradient on a given surface while providing structural support during fabrication. For a 100 micron thick wall chamber, there was high stress on the top surface while for a 250 micron thick wall, the average stress on the same surface was significantly lower.

It would be desirable to have a large number of fins to provide the highest possible surface area for cooling the adsorbent material. However, a large number of fins would also reduce the available volume for adsorbent material and thus reduce the vacuum performance of the pump. It was found that a pump having four fins, each with a thickness of approximately 100 microns, gave the best balance between these competing parameters. Additionally, it was found that fins tend to having rounded ends as shown in FIG. 4, had reduced stresses. Accordingly, cooling fins 10 having rounded ends are preferred in the micro-sorportion pump 1 of the present invention as illustrated in FIG. 4.

The second space preferably has a high aspect ratio, with the range of the aspect ratio being 100:5-25.

An adsorbent (not shown) is contained in the second space 8 and can be a carbon molecular sieve or a zeolite molecular sieve. As a carbon molecular sieve, Spherocarb® (registered tradename of Phase Separations) can be used. Additionally, silicon dioxide-based nanoporous structures can also be used in the present invention. However, carbon molecular sieves, such as Spherocarb®, are preferred. The silicon dioxide-based nanoporous structure can be fabricated using the methods suggested by O. D. Velev et al in Nature, Volume 389, 447-8, 1997. The amount and type of coolant depends on the amount of vacuum desired and the gaseous components of the atmosphere to be adsorbed by the adsorbent and is readily determinable by one of ordinary skill in the art.

A suitable liquid coolant, such as liquid nitrogen, is contained in the chamber 7 formed between the first and second continuous vertically extending walls, as shown in FIG. 1. Instead of a liquid coolant, a micropeltier device having a cooling surface in contact with the outer surfaces of the second continuous vertically extending wall 6 can be provided in the chamber 7.

In an alternative embodiment of the present invention exemplified in FIG. 3, the micro-sorption pump comprises a base wall 2 made of a thermally conductive material, a continuous vertically-extending wall 6 made of a thermally conductive material provided on the base wall and enclosing a space therein for containing an adsorbent material, cooling fins 10 provided on the continuously vertically extending wall 6 and extending into a space 8 for containing an adsorbent material, and a micropeltier unit 11 which contacts with and cools the base wall 2 and the continuous vertically-extending wall 6.

As shown in FIGS. 2-4, a coupling sleeve 12 can be used to connect the microsorption pump 1 to a microdevice 17, such as a micro mass spectrometer, micro chromatographs, micro crack sensing devices which can be used in microcrack detection of large aeroplane and other structures and chemical or gas sensing lab-on-chip devices, which need evacuation prior to their use. The coupling sleeve 12 can comprise an annularly-shaped member which is made of a suitable thermally insulating material which is attached to the outer surface of the microdevice 17 and is adapted to fittingly engage with the outer surface of the micro-sorption pump 1. A hollow cylindrically shaped piercing member 16 can be provided which engages with a microchannel 20 provided in the microdevice 17 and is adapted to pierce a seal or membrane 15 which seals the space 8 containing the adsorbent material from the outside. The piercing of the seal 15 by the member 16 brings the microchannel 20 into fluid communication with the adsorbent-containing space 8 and allows the micro-adsorption pump 1 to lower the pressure in the microchannel 20.

In the embodiment of the invention illustrated in FIG. 1, the second space 8 is approximately 1 mm in diameter and 1 mm high in a preferred embodiment. The aspects ratios of these structures can be easily modified to suit the requirements of a given application or process. In this preferred embodiment, the diameter of the chamber 7 is approximately 2 mm. The micro-sorption pump 1 of the present invention can be combined with a liquid flow system to introduce and remove the liquid coolant from the chamber 7 and can be integrated into a series of micro-sorption pumps in order to meet vacuum requirements of larger systems. In such a case, these micro-sorption pumps can be connected from beneath by designing microfluidic channels within the substrate before fabricating the pump structures on the top. These interconnected channels can be used for evacuating the given microdevice, through one external connection only.

Although preferred embodiments of the present invention have been described herein, they are presented as Examples of the present invention and the present invention is by no means limited thereby. The present invention is also intended to encompass the micro-absorption pump in different shapes and configurations as long as the basic operating features of the present invention are not departed from. 

1. A micro-sorption pump comprising: a base wall made of a thermally conductive material; a first continuous vertically-extending wall made of the thermally conductive material provided on the base wall and enclosing a first space therein; a second continuous vertically-extending wall made of the thermally conductive material provided on the base wall inside the first wall and enclosing a second space therein, the first and second walls defining a chamber therebetween; cooling fins provided on the second wall and extending into the second space; and an adsorbent material contained in the second space.
 2. The micro-sorption pump of claim 1, additionally comprising a means for cooling the adsorbent contained in the chamber.
 3. The micro-sorption pump of claim 1, wherein said base wall has a circular configuration and said first and second walls have annular configurations.
 4. The micro-sorption pump of claim 2, wherein the means for coding the adsorbent material is a liquid.
 5. The micro-sorption pump of claim 4, wherein said liquid is liquid nitrogen.
 6. The micro-sorption pump of claim 2, wherein the means for coding the adsorbent material is a peltier device.
 7. The micro-sorption pump of claim 1, additionally comprising a rupturable seal for sealing the second space from the outside.
 8. The micro-sorption pump of claim 1, wherein said thermally conductive material is selected from the group consisting of copper, nickel, copper-nickel alloys, nickel-iron alloys and silicon.
 9. The micro-sorption pump of claim 1, wherein the cooling fins have rounded ends.
 10. The micro-sorption pump of claim 1, wherein the adsorbent material is selected from the group consisting of a carbon molecular sieve and a zeolite molecular sieve.
 11. The micro-sorption pump of claim 1, wherein the second space has an aspect ratio of from 1,000:50-250.
 12. A micro-sorption pump comprising: a base wall made of a thermally conductive material; a continuous vertically-extending wall made of the thermally conductive material provided on the base wall and enclosing a space therein for containing an adsorbent material; cooling fins provided on the continuous vertically extending wall and extending into the space for containing an adsorbent material; an adsorbent material contained in the space; and a micropeltier unit for cooling the continuous vertically-extending wall.
 13. The micro-sorption pump of claim 12, wherein the micropeltier unit comprises a chamber having walls which are in thermal contact with the continuous vertically extending wall.
 14. A microsystem comprising a microdevice integrated with the micro-sorption pump of claim
 1. 15. The microsystem of claim 14, wherein the microdevice is a mass spectrometer.
 16. The microsystem of claim 14, wherein the microdevice is selected from the group consisting of a micro chromatograph, micro crack sensing device and a chemical or gas sensing lab-on-chip device.
 17. The microsystem of claim 14, additionally comprising a sleeve for mounting the micro-sorption pump to the microdevice.
 18. The microsystem of claim 14, wherein the micro-sorption pump comprises a seal for sealing the second space from the outside and the microdevice has a perforating member for breaking the seal and bringing the second space into communication with a microchannel contained in the microdevice.
 19. A microsystem comprising a microdevice integrated with the micro-sorption of claim
 12. 